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Summary of Significant Accounting Policies - Variable Interest Entities (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
USD ($)
joint_venture
Dec. 31, 2019
USD ($)
Variable Interest Entity [Line Items]    
Number of joint ventures considered as VIEs | joint_venture 4  
Asset $ 9,965,798 $ 9,609,370
Liability 8,110,760 8,380,434
Variable Interest Entity, Not Primary Beneficiary    
Variable Interest Entity [Line Items]    
Asset 28,969 28,579
Liability 0 0
Maximum Exposure to Loss 28,969 28,579
VIEs' assets $ 175,547 $ 119,051
JV building microwave communication networks in US, Europe and Asia    
Variable Interest Entity [Line Items]    
Number of joint ventures | joint_venture 2  
JV building microwave communication networks in US and Asia    
Variable Interest Entity [Line Items]    
Ownership of equity of JV held be each investor (as a percent) 10.00%  
JV building microwave communication networks in US and Europe    
Variable Interest Entity [Line Items]    
Ownership interest 50.00%  
JV offering derivatives trading technology and execution services    
Variable Interest Entity [Line Items]    
Ownership interest 10.00%  
JV developing a member-owned equities exchange with the goal of increasing competition    
Variable Interest Entity [Line Items]    
Ownership interest 14.10%